نشریه مهندسی مکانیک مدرس
سال پانزدهم شماره 7 (مهر 1394)

The objective of this paper is to analyze the nonlinear vibrations of simply supported pseudoelastic shape memory alloy (SMA) cylindrical shell under harmonic internal pressure based on Donnell-type classical deformation shell theory. The pressure is a function of time and space. The behavior of pseudoelastic SMA is simulated via the Boyd–Lagoudas constitutive model numerically implemented by the Convex Cutting Plane Mapping algorithm. The Hamilton’s principle is employed to obtain the equations of motion. Differential Quadrature Method (DQM) and Newmark time integration scheme are applied to get the time and frequency responses of the cylinder. Also, the natural frequencies of the shell are obtained for the case of pure austenitic phase to compare the frequency response of the present nonlinear system (phase transformation –induced material nonlinearity) with the linear one around them. Results indicate that the strength of the material will decrease during the phase transformation. This fact is proved by the softening behavior observed in the frequency response of the system due to the phase transformation. Further, the pure austenitic phase shell is simulated in ABAQUS to verify the results. A good agreement is found between two outcomes.

Mechanical properties of polymeric materials are significantly sensitive to the loading rate. Therefore, it is necessary to develop a dynamic constitutive model to investigate their strain rate dependent mechanical behavior. In this study, first by conducting torsion experiments the shear behavior of neat and reinforced epoxy with carbon nano-fibers (CNFs) was studied experimentally. Then, the Johnson-Cook (J-C) model has been modified to be able to model the shear behavior of neat polymers. The strain rate effects on elastic behavior of polymers were considered by introducing a material equation. Then, by combining the modified Johnson-Cook (MJ-C) model with a micromechanical model (Halpin-Tsai model) and using pure polymer experimental tesults and mechanical properties of carbon nano fiber, the strain rate dependent mechanical behavior of polymers reinforced with CNFs at arbitrary strain rates and volume farction of carbon nanofiber has been predicted. The new model presented in this research is called as the dynamic-micromechanical constitutive model. The predicted results for the neat and nano-phased polymers were compared with conducted and available experimental results. It has been shown that the present dynamic constitutive model can predict the strain rate dependent mechanical behavior of polymeric materials with a good accuracy.

The aim of this paper is to investigate the green density, the percentage of porosity and the density distribution of materials which have been produced by powder compaction procedure under low rate impact loading by using drop hammer both experimentally and analytically. Effect of grain size and different level of energy on density is carried out in the experimental section. In this regard, the effect of different level of energy are investigated by changing mass and height of hammer. The analytical section presents a relation for green density considering a small element of compacting piece and using equilibrium equation, continuity equation and Levy-Mises equation. Using the statistical analysis leads to investigation of the effect of grain size and friction coefficient simultaneously as two impressive factors on analytical green density. In the next step, the percentage of porosity and density distribution was calculated analytically and compared with experimental values. The satisfactory accordance between Experimental results and analytical ones validates the presented analytical results. Also by applying two constant quantities, shape factor and work hardening in analytical relations, the effect of these factors on percentage of porosity and density distribution of products have been investigated.

In statistics, Entropy is a measure of disorder of time series. Entropy is used in physiologic for signal analysis. In physiologic science, Entropy is used for performance analysis of body organs such as heart and brain. Epileptic patients have been diagnosed by this technique. In this paper for the first time, Entropy is used to determine the health condition of mechanical systems. A special kind of Entropy, namely Permutation Entropy is used for this purpose.To perform the experiment an apparatus consisting of a motor coupled with a shaft has been designed and manufactured. Vibration signals from supporting bearing of this system in different shaft states namely healthy shaft, and shafts with 3, 5 and 7 mm crack were gathered with a vibration data analyzer. The vibration were taken from sensors mounted on bearing supports of the shaft. Shaft was subjected to a constant bending moment. The vibration signals were preprocessed by permutation Entropy method. Nine different features were extracted from the Entropy signals which are fed to an Adaptive Neuro Fuzzy Inference System (ANFIS). The designed ANFIS was capable of classifying different shaft states with an overall %96 percision.

The prediction of distillation zone is very important in steam turbine blades and steam nozzles. In identification of distillery with equilibrium method, as the steam flow contacts the two-phase dome, the second phase formes and flow properties will pass the distillery without any jumping, therefor after crossing the saturation curve, the droplet formation transpires, but in non-equilibrium method by a sudden increase in pressure, called “condensation shock” a discontinuity in the flow characteristics is seen and after crossing the saturation curve, the formation of droplets starts. In this paper, numerical analysis of a vapor-liquid two-phase transonic flow in a convergent-divergent nozzle with and without shock is investigated. Effects of stagnation temperature at nozzle inlet, viscosity and geometry is studied using thermodynamic equilibrium and non-equilibrium methods and results compared with experimental datas. Roe numerical method is used for vapor-liquid two-phase flow numerical solution. The main properties of the flow at the boundary of elements is extrapolated by MUSCL third order acuracy and time discretization is performed using Lax-Wendroff explicit two-step method of second order accuracy. It is observed that the results of non-equilibrium solution, has more correspondence to experimental results and Condensation starts earlier in the nozzle with further expansion rate. By increasing the temperature at nozzle inlet, the place at which condensation starts goes forward. Also in comparision with non-viscous flow, the shock location in viscous flow comes closed to the throat.

The problem of two wheeled self-balancing robot is an interesting and challenging problem in control and dynamic systems. This complexity is due to the inherent instability, nonholonomic constraints, and under-actuated mechanism. Dynamical model of two wheeled self-balancing robot can be presented by a set of highly coupled nonlinear differential equations. Authors, previously, developed the modified dynamical equations of the robot. The governed equations have some differences with the commonly used equations. The main difference is due to the existence of a nonlinear coupling term which is neglected before. In this paper we used an adaptive sliding-mode controller based on the zero dynamics theory. The controller objective is to drive the two wheeled self balancing robot to the desired path as well as to make the robot stable. By some simulations the behavior of the robot with the proposed controller is discussed. It is shown that if the nonlinear coupling term is ignored in designing the controller, the controller cannot compensate its effect. Using Lyapunov theorem and the invariant set theorem, it is proved that the errors are globally asymptotically stable.

In this paper flutter phenomena for a cropped wing with an external store using numerical and experimental methods in a subsonic and incompressible flight regime has been studied. Wing structure was modeled base on von Karman plate theory. A 3D time domain unsteady vortex lattice method was used for wing aerodynamic model and a slender body aerodynamic theory was used for store aerodynamic model. Finally, the aeroelastic governing equations with considering vibratory wing motion has been solved. The experimental tests were performed in an incompressible subsonic wind tunnel. Comparison of experimental results with theoretical analysis shows good agreement with each other especially in calculation of aeroelastic behavior of the wing. In continue, the effects of some parameters such as wing thickness, wing aspect ratio, store position, weight of the store, aerodynamic of the store, store vertical distance from under wing, and center of mass of the store on both flutter speed and instability boundary of the wing have been studied analytically and experimentally. The results show with both increasing aspect ratio and decreasing wing thickness, flutter speed will be decreased. Moreover, change in store position effects on flutter speed of the wing/store configuration. Aerodynamic of the store has no significant effect on flutter speed of the wing/store configuration and increasing store weight leads to increasing flutter speed. Change in center of mass of the store influences on flutter speed.

Inducing compressive residual stress is one of the methods of improving fatigue life in metallic components. There are numerous and various methods for inducing compressive residual stress, such as shot peening and deep rolling. One of the most recent and most advanced methods for inducing compressive residual stress in industrial components is Laser Shock Peening (LSP). LSP is a relatively new and complex process, therefore, vast experimental investigations are needed for better understanding the process. For this purpose in the present work, an Nd: YAG Laser with 1200mJ of energy per pulse was used to investigate the LSP process experimentally on Al 6061- T6 alloy. The effect of process on the hardness beneath the surface, the microstructure, and the surface roughness was studied. In addition, in order to investigate the effect of the LSP process on a notch, a notched sample was treated using the LSP process. The results showed that the process could increase the hardness of the material up to 1000μm below the surface. Furthermore, the results showed that the surface roughness would slightly get increased, while this increase could be limited by properly selecting the process parameters. The LSP process of the notched sample showed that this process could lead to the growth of cracks in such samples.

Careful and numerical analysis eccentrically stiffened shells in the industry is a major step forward in the design of these shells. In this paper, a careful analysis of post-buckling behavior of eccentrically stiffened FGM thin circular cylindrical shells is surrounded by an elastic foundation and external pressure is presented. The two parameter elastic foundation based on Winkler and Pasternak elastic model is assumed. Stringer and ring stiffeners are internal. Shell properties and eccentrically stiffened are FGM. Fundamental relations and equilibrium equations are derived based on the smeared stiffeners technique and the classical theory of shells and according to von- Karman nonlinear equations. The three-term approximation for the deflection shape, including the pre-buckling, linear buckling shape and nonlinear buckling shape was chosen that using the Galerkin method, the critical load and post-buckling pressure-deflection curves is calculated. The effects of different dimensional parameters, buckling modes, volume fraction index and number of stiffeners are investigated. Numerical results show that stiffeners and elastic foundation enhance the stability of the shells. Increasing the shell thickness, reducing the volume fraction index, raising the number of Stringer and ring stiffeners and applying foundation elastic, causes the critical buckling load is increased, too.

In this study, by modeling van der Waals (vdWs) interactions based on the Lennard-Jones potential function interlayer tensile-compressive and shear moduli of bilayer graphene sheets are analytically calculated. To this end, by varying potential depth parameter which shows the strength of vdWs interactions a new model is presented for calculating interlayer in-plane and out-of-plane moduli for two different stacking patterns. In order to determine the interlayer vdWs moduli, a small flake of monolayer graphene is sliding on a large monolayer graphene substrate and accordingly variations of vdWs forces as well as the interlayer shear and normal strains are recorded. The relative displacements of layers cause linear strain and stress. In the model, bilayer graphene geometry (being armchair or zigzag, and stacking pattern) and potential depth parameter are two important parameters for determination of vdWs moduli. The accuracy of the method is verified by comparing the present results with those reported in literatures. Finally, close-form relations for interlayer tensile-compressive and shear moduli of vdWs interactions versus the depth potential parameter are presented for ABA and AAA stacking patterns as well as zigzag and armchair directions. It is observed that the interlayer moduli have linear relation with the potential depth parameter.

Laser processes are widely used for surface properties improvement of parts and components. Laser cladding, using laser as heat source, is an innovative method that can be used for improving surface properties. In this investigation, preplaced technique of laser cladding process of WC powder on Inconel 718 using pulsed Nd:YAG laser is studied. A number of parameters affect the energy density in the process, ultimately affecting the clad quality and geometry. In this study, laser average power, pulse width, focal distance, scanning speed and pre- placement factors are input parameters, while clad dilution, fusion depth, porosity and number of cracks are outputs. It is observed that hard phases are formed on Inconel surface in-situ by laser beam radiation and small changes are made in the substrate properties in a limited zone. Experimental results reveal that energy density of the laser beam is the most important factor affecting the number of cracks in laser cladding process. Also, dilution and porosity are highly affected by laser average power. Furthermore, in multi-pass laser cladding process, 50% overlap between adjacent passes has good results. Experimental results show that by concise arrangement of input parameters, one can achieve to an optimum clad layer. Thus, laser cladding process can efficiently supersede other conventional methods.

The presence of environmental and measurement noises and ignoring the input effects are the main sources of error in system identification using ambient vibration test results. Therefore, reducing uncertainty or noise levels from the records has always been one of the main goals of the new techniques in the field of ambient vibration. Among the modal analysis techniques, stochastic subspace identification is considered as a powerful technique. In this study, the modal analysis method based on canonical correlation analysis in stochastic subspace is presented that identifies dynamic properties in optimized space instead of data space by extracting ortho-normal vector of data space. The advantage of this method, due to the nature of canonical correlation analysis, is lower noise which results in greater accuracy in estimating modal properties. Moreover, the presented process is faster due to the smaller space of identification compared to the previous methods. To validate the proposed method, an analytical model of two-dimensional frame excited under Elcentro earthquake acceleration and also the results of ambient vibration tests carried out on the Alamosa Canyon Bridge are used. The results indicate that this method eliminates more noise than other subspace methods and moreover it is faster in solving practical problems. The computation of dynamic properties, natural frequencies and mode shapes, of Alamosa Canyon Bridge with 30 sampling sensors, space matrix size of 750 and 50 excited modes are carried out in less than 150 seconds with a quad-core 2.30 GHz processor.

Fluid flow through channels and ducts in nano scales is an important issue which needs numerical simulations for better analysis of fluid behavior because of the limitations of experimental methods. Hence, in the present study Molecular Dynamics simulation is used as a precise method for molecular scale problems to investigate fluid behavior. This method which is based on Newton’s second law, is applied to investigate liquid Argon flow in steady Couette flows through smooth and rough nanochannels. Using LAMMPS software, were performed simulation. In the present study, the fluid velocity and fluid slip in steady Couette flows were obtained to investigate various effects including: wall velocity, channel height, wall density, fluid-wall interaction, and surface roughness with different shapes such as rectangular and triangular in different dimensions. Based on the results, an increase in wall velocity increases the fluid slip velocity. For velocity constant values, an increase of channel height will decrease the fluid slip velocity. In steady Couette flow, decrease of wall density will result in decrease of fluid slip velocity. Reducing the energy parameter between fluid and wall will increase the fluid slip velocity and on the other hand, decreasing the fluid-wall length parameter will decrease the fluid slip velocity. The rectangular and triangular roughness at the bottom wall reduces the fluid slip velocity, and an increase of roughness height will further decrease the fluid slip velocity.

Applications of aluminium and magnesium castings have been increased, as a result of increasing demand for the light weight components in various sectors of industries, in recent years. In this work an Al/Mg bimetal was prepared by casting Al melt into a cylindrical Mg bush, with 35 mm height and 76 and 84 inner and outer inner diameter, rotating at 1200 and 1600 revolutions per minute (rpm), 0.9, 1.6 and 2.7 melt-to-solid volume ratio and 30, 120, 150 and 200 oC preheating temperature, respectively. Vertical centrifugal casting process was selected for producing samples. In this process melt is under effect of centrifugal, coriolis and gravity forces during filling. Difference between shrinkage of Al and Mg led to the formation of mechanical bond in the interface. The results of scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analysis showed that concentration gradient changes from Mg to Al side in such a way that three sub layers including Al3Mg2 and Al12Mg17 intermetallics plus eutectic microstructure (Al12Mg17 and δ), were formed, based on aluminium and magnesium phase diagram, in the interface.

Finding a stable trajectory is amongst pivotal subjects for bipeds, which are a type of legged-robots. To mimic human gait, biped robots are basically complex because of having numerous Degrees of Freedom. The main goal of this paper is to design a stable trajectory of gaits for a 9 links robot via Zero Moment Point stability criteria. The robot used in this paper is composed of 16 active joints with two toe joint. One of the aspects of human walking to design longer strides is to use a status in which the foot is rotated about its toe joint. Here, a gait type is utilized whereas the entire sole of the support foot firstly touches the ground then rotates about its toe axis as an active joint. To achieve an initial admissible equation for robot motions, a constraint is used for initial guess of pelvis motion. Then, by using a novel algorithm, the trajectory of the joints is calculated. Finally, by considering Zero Moment Point and a trial-error algorithm, the desired trajectory of the biped robot is obtained.

One of the quality characteristics of welded joints in gas metal arc welding (GMAW) is weld height (WH). This paper highlights an experimental study carried out to develop a model using artificial neural network (ANN), to predict WH in GMAW in the presence of TiO2 nano-particles. For developing the model, the arc voltage, welding current, welding speed, percentage of Ar in Ar-CO2 mixture and thickness of TiO2 nano-particles were considered as input parameters and WBH as the response. A Doehlert design matrix was employed in the experiments to generate experimental data. The ANN model was developed and validated by conducting five extra runs. The remarkable outcome of this study is the mechanism of arc constriction due to interacting effects between welding input parameters and TiO2 nano-particles. Moreover, the results showed that increasing thickness of TiO2 nano-particles up to almost 0.9 mm increased weld height whereas, its further increase up to 1.0 mm, decreased weld height subsequently. In fact, this variation in weld height could be due to thermal dissociation of TiO2 nano-particles and CO2 releasing oxygen onto weld pool surface, which influenced its surface tension and consequently, changed direction of the Marangoni convection of fluid flow in weld pool and as a result, affected WH. For ANN technique, MSEtrain=0.0066, MSEvalidation=0.0063 and MSEtest=0.0093. Finally, it is to be concluded that ANN is an accurate technique for predicting weld height.

Oil journal bearings are one of the most common parts of high load carrying rotating machine. Stability of these bearings can be affected by various stimulus such as changes in loading and lubrication conditions. Therefore, identification of the dynamic response of journal bearings can improve the control and fault detection process of rotor-bearings systems and prevent them from placing in critical operation condition. Since past, the mass unbalance of rotor is proposed as an effective factor on the dynamic behavior and long life of bearings. For this reason, in this research the effects of this parameter on the stability of hydrodynamic two lobe noncircular journal bearing with micropolar lubricant is investigated based on the nonlinear dynamic model. To achieve this goal, the governing Reynolds equation is modified with respect to micropolar fluid theory and the equations of rotor motion are derived considering the mass unbalance parameter. The static and dynamic pressure distributions of the lubricant film and the components of displacement, velocity and acceleration of the rotor are obtained by simultaneous solution of the Reynolds equation and the equations of rotor motion. Investigation of results in terms of dynamic trajectory, power spectrum, bifurcation diagram and Poincare map show that the dynamic behavior of two lobe bearings appears in different manner with variation of mass unbalance of rotor. The response of analyzed dynamic system include converge oscillations to the equilibrium point, periodic, KT periodic and quasi periodic behavior and also divergent disturbances which leads to collision between the rotor and bearing.

Polymeric Due to high strength to weight ratio of polymeric composites and their directional properties, they are extensively used in engineering, particularly in aerospace industry. However, the difference in material properties of composites makes their failure prediction complicated especially under cyclic loading. Present study is carried out to develop a new method for estimation of the intralaminar fatigue damage of fibrous composites based on continuum damage mechanics. In order to include the influence of microscopic defects in three material orientations, three internal material state variables namely damage variables are defined in thermodynamics framework. By considering a 3-directional damage propagation, suggested model is able to make a good prediction of laminated composites fatigue life. To achieve this, a closed form solution by energy method in framework of thermodynamics is presented. The solution is in a way to include the differences in damages of various directions yet maintaining the independency on the layup. The model is implemented in ANSYS software by using a user material code (Usermat). This method gives us an advantage to estimate the fatigue life of any laminate with arbitrary layup under different loading conditions only by having static and fatigue properties of a unidirectional ply. Characterization of constants of model is presented and they are also determined for a certain composite material. Comparison between the predicted results of proposed model and the available experimental data verifies the great precision of the model.

This article investigates experimental study of the flow field on a blunt airfoil. For this purpose, PIV technique based on instantaneous flow structures is used in order to view and two dimensional investigation of flow field around unmodified and blunt airfoil and at different times. This study is performed on flows at very low Reynolds number(Reynolds number lower than 4500). This flow regime is very similar to dominant condition on micro air vehicles (MAVs). In order to validate the method used in this study, flow field around cylinder is considered and in continue, instantaneous and mean velocities fields, streamlines and mean vortices field around unmodified and blunt airfoils are obtained. The results show that there are prominent differences on the structure of wake around airfoils and sizes of separation region for blunt and simple airfoils. Meanwhile separation of the flow for both blunt and simple airfoils at this very low Reynolds number, is occurred at angle of attack 5 (at low angle of attack). Also generation of vortex at wake region and their position and circulation at different times, are discussed.

The purpose of this paper is design, construction and the control of a two-wheel self-balancing robot. For this purpose firstly, a literature study is carried out on the history of manufactured self-balancing robots and the researches which have been done so far in this area are reported. In addition, the robot chassis with consideration of the size and material is analyzed; and the dynamic equations of the robot are computed according to the designed chassis. Then, the robot inertial parameters are measured through different experimental tests and these parameters are used in the equations. Also, the derived equations are simplified and the transfer functions are evaluated for considering the stability of the robot. In this self-balancing robot, the simplified Kalman and complementary filters are used for identifying of the bias angle from the vertical position by combination of data obtained from accelerometer and gyroscope sensors. The PID controller and the robot transfer functions are simulated in MATLAB software. Then, the controller gains are obtained for the stability of the constructed robot. These gains are computed by PID tuning toolbox of MATLAB software as well as theoretically, and the results in each method have been compared with each other. Finally, the robot control electronic circuit is designed for analyzing the results through AVR microcontroller, while angle identification sensor is used.

Cement rotary kilns are extensively used to change raw material into clinker. This is a complex process and consists of many different phenomena such as bed material reactions, gas phase turbulent combustion and radiation in a rotary drum, and thermal-mass interactions between them. Using CFD, the two-dimensional numerical simulation of cement rotary kiln was performed in the present study. This model included gaseous fuel combustion, bed material reactions, and radiation heat transfer in the kiln. Using this model and parallel processing network, combustion models (PaSR, EDM and mixture fraction) in the cement kilns are investigated. Due to the high Damkohler number in the cement kiln (0.7<Da<17), selecting the appropriate combustion model is difficult. Among the combustion models that were studied, it was found that the PaSR model is the slowest and mixture fraction model is the fastest model whereas the both models predict physics well.

One of the most important issues in the review of cold roll forming process of metals is estimation of required torque. The optimum production line can be designed by determining the effective parameters on torque. Some of these parameters are sheet material and thickness, bending angle, lubrication conditions, rolls rotational speed and distance of the stands. The aim of this study is to predict amount of required torque considering the factors influencing torque, including thickness, yield strength, sheet width and forming angle using artificial neural network. So the forming process was 3D simulated in a finite element code. Simulation results showed that with increase of yield strength, thickness and forming angle, applied torque on rolls will increase. Also the increase in sheet width -assuming constant web length- will decrease the torque needed for forming. The effects of thickness and sheet width were experimentally investigated which verified the results obtained by finite element analysis. A feed-forward back-propagation neural network was created. The comparison between the experimental results and ANN results showed that the trained network could predict the required torque adequately.

In recent years the use of metallic bipolar plates for fuel cells is considered. Several studies have been conducted on the various methods of forming these plates. Most of this research has been done on the serpentine flow fields. While in some cases that the pressure drop is important factor, the pin-type flow fields shows good performance. In this research, hydroforming of metallic bipolar plates with circular pin-type pattern from stainless steel 304 with 0.11mm thickness is investigated experimentally and numerically. For this purpose, the effect of geometrical parameters such as the die wall angle, the die chamfer dimension, the depth-to-width ratio of the die, and forming pressure on the profiles, filling percent, thickness distribution and thinning percent of the formed parts are investigated. In this regard, two dies with wall angle of 0 and 15 degree were prepared. Then experimental tests were done at different pressures. After performing the required tests, the results show that the die wall angle leads to a more uniform thickness distribution and higher precision of the parts profile. Also the suitable range of die geometrical parameters was determined.

Hartmann-Sprenger tube is a device in which an under-expanded jet enters a closed-end tube which is placed in a specific distance from the nozzle. Because the tube is closed at its end and the jet flow continues, the oscillating flow could produce an intensive heat in the taped gas inside the tube. The present study focuses on the numerical analysis of the flow in various phases of the oscillatory process in a sample resonance tube. First, the numerical model is generated with respect to the physical knowledge of the problem. Then, the problem is solved numerically and the results will be discussed in various steps of the process. Numerical results are in good agreement with the experiments. The analysis shows that the process consists of two major phases; the transient flow and semi-steady flow. In the transient phase, the changes are more violent than the second phase. On the other hand, the amount of heat generation and dissipation is different in these two phases. In Fact, the most important factor in the heat generation process is the compression waves passing through the points inside the tube; which is repeated periodically. Also, the main mechanisms for heat dissipation in the tube are the mass displacement and the heat transfer from the walls. Finally, the flow analysis will lead to increasing the insight for the flow and heat generation mechanism in a Hartmann-Sprenger tube and decreasing the uncertainties.

Present research has been done with the aim of investigating hydrodynamic behavior of slug flows in a transparent acrylic tube with inner diameter of 40 mm and height of 3.33 m. The vertical experimental system constructed in Two-Phase Flow lab in Tarbiat Modares University was used to perform needed experiments. By using image processing technique, recorded movies of flow structures were analyzed and some important characteristics of slug flow such as length and velocity of Taylor bubbles and liquid slugs between them were extracted. In addition, the average path line of Taylor bubble nose was computed in a proper range of the tube length. The acquired probability density functions show that there is a direct relationship between the increasing of Taylor bubble length and liquid slug length moving after it. Also rising velocity of shorter Taylor bubbles is more than longer ones. Results show that bubble nose does not violate ± 20 % around the center line of the tube. An experimental correlation based on the Taylor bubble velocity and total superficial velocities of phases is presented which shows that the famous Nicklen correlation does not work well for this tube diameter.

Traumatic brain injury (TBI) has long been known as one of the most unspecified reasons for death around the world. This phenomenon has been under study for many years and yet questions remain due to its physiological, geometrical and computational complexity. Because of the limitations in experimental study on human head, the finite element human head model with precise geometric characteristics and mechanical properties is essential. In this study, the visco-hyperelastic parameters of bovine brain are extracted from experimental data and finite element simulations which are validated by experimental results. Then a 3D human head including brain, skull, and the meninges is modeled using CT-scan and MRI data of a 30-years old human. This model is named “Sharif University of Technology Head Trauma Model (SUTHTM)”. After validating SUTHTM, the model is then used to study the effect of G acceleration. Damage threshold based on consciousness in terms of acceleration and time duration is developed using HIC and Maximum Brain Pressure criteria. Results revealed that Max. Brain Pressure ≥ 3.1 KPa and HIC ≥ 30 are representative of loss of consciousness. Also, 3D domains for the loss of consciousness based on Max. Brain Pressure and HIC criteria are developed.

Extended finite element method (X-FEM) has been recently emerged as an approach to implicitly create a discontinuity based on discontinuous partition of unity enrichment (PUM) of the standard finite element approximation spaces. Despite numerous progresses in mesh generating updating of finite element mesh during crack propagation remain extremely heavy and difficult. This problem becomes more complicate, when there are many discontinuities in the finite element domain. However, the extended finite element method (X-FEM) in the combination with level set method (LSM) could overcome this cumbersome issue. In this contribution, predefined cracks and internal boundaries are created using level set functions and also the effects of soft/hard inclusions (interfaces) and voids are considered on crack propagation schemes. In fact, the interaction of crack and heterogeneities are considered. The level set functions are utilized to represent the locations and the evolutions of internal interfaces. In addition, the stress intensity factors for mixed mode crack problems are numerically calculated by using the interaction integral method. Different crack growth paths are simulated automatically for different oriented edge and center cracks and the interactions of internal boundaries on crack propagations are shown. All numerical examples are demonstrated the flexibility and capabilities of X-FEM in the applied fracture mechanics.

In this paper, elastic-plastic buckling of a thick rectangular plate has been investigated based on both Incremental (IT) and Deformation (DT) plasticity theories. Uniform biaxial edge traction was assumed as the plate loading while simply supported as the boundary conditions. Integral uniqueness criterion has been minimized to determine the critical buckling traction. Based on Rayleigh-Ritz method, a linear combination of polynomial base functions, which satisfy the geometrical boundary conditions, has been used as the trial functions for rotations and transverse deflection. To validate the analysis, the results for the Mindlin plate theory have been compared with the previously published results and a very close agreement has been observed. Then the effects of thickness ratio, aspect ratio and also different biaxial traction ratios on the buckling traction have been investigated. The results show that for the problem considered here, very close critical buckling traction is predicted by the both Mindlin and sinusoidal plate theories. This implies that Mindlin plate theory is sufficiently accurate to predict critical buckling traction in this problem. Moreover when the loading is gradually changed from biaxial into uniaxial compression or when the thickness-ratio is increased, the difference between the two theories is also increased. Also for compression-tension loading case, the critical buckling traction predicted by deformation theory is much less than the incremental theory.

Attitude control of spacecraft is one of the most important issues in aerospace. Different control method for this purpose were proposed that each of these controllers have different behavior against disturbances. One of these methods is backstepping which is divided to adaptive and nonadaptive techniques. Since the spacecraft equations are nonlinear, linear control methods will not work in this case so the nonlinear control methods should be used. The modular adaptive backstepping method is a nonlinear adaptive control method and has a parameter update law that we can use different estimators to estimate system’s unknown parameters in this method. It is also possible to reduce the differentiation load of virtual control laws by using the command filtering method. In this paper, contrary to other works which mostly use discrete command filtering method, we use continuous command filtering method which the natural frequency and damping coefficient can be determined to bring down the computation of the time derivatives of virtual controls laws to differentiation. In this paper, after deriving spacecraft equations in terms of Modified Rodrigues Parameters, we design two stable attitude controllers for spacecraft using standard and command filtered modular adaptive backstepping methods and prove the stability of system using the Lyapunov theory and then simulation results of these controllers are compared with each other. Simulation results show good attitude tracking accuracy and success of adaptive backstepping method in having robustness against disturbance torque is proved

In this article numerical simulation of electroosmotic flow in heterogeneous microchannel is performed using approximate model of Helmholtz-Smoluchowski in which the effect of electric field on the fluid flow is applied through a slip boundary condition. Solving the concentration equation, the mixing performance of microchannels with heterogeneous zeta-potential is studied both qualitatively and quantitatively. This study shows that combining the electroosmotic and pressure-driven flows in a single microchannel with proper arrangement of the heterogeneities can easily lead to design of electroosmotic micromixers with adjustable mixing performance. The mixing behavior of such micromixers is dominated by the arrangement of zeta-potential distribution as well as the applied external pressure drop. In this article we introduced relative mixing performance and mixing capacity rather than well-discussed factor of mixing performance in order to perform a thorough analysis of mixing. Using these factors, it is found that presence of heterogeneities has a small augmentation on mixing performance when the pressure drop is extremely small or large. Therefore, performance of micromixers with combined flow of electroosmotic and pressure-driven has an optimum point. Furthermore, it is seen that asymmetric level of the charge pattern is more effective on the mixing performance compared to absolute values of wall charges. This promises proper mixing even when surfaces with moderate zeta-potential are used in micromixer.

Materials are often damaged during the process of detecting mass fractions by traditional methods. In this work, acoustic emission (AE) technology combined with wavelet packet analysis is used to evaluate the mass fractions of graphite/ epoxy composites. Attenuation characteristics of AE signals across the composites with different mass fractions are investigated. The AE signals are decomposed by wavelet packet technology to obtain the relationships between the energy and amplitude attenuation coefficients of feature wavelet packets and mass fractions as well. Furthermore, the relationship is validated by test samples. The results show that the lower proportion of graphite will correspond to the less attenuation. The attenuation characteristics of feature wavelet packets with the frequency range from 125 kHz to 171.85 kHz are more suitable for the detection of mass fractions than those of the original AE signal. The error of the graphite mass fraction calculated by the feature wavelet packet (1.9%) is lower than that of the original signal (4.75%). Therefore, the AE detection base on wavelet packet analysis is an ideal NDT method for evaluate mass fractions of composite.

In this study, bubble dynamic and the mechanisms to cause net vapor generation (NVG) were explored experimentally in a rectangular vertical upward subcooled flow boiling under atmospheric pressure, and new results was found on various conditions of surface wettability. In the course of observation, two different vapor bubble behavior were observed and in low void fraction region new mechanism for incipience of net vapor generation was proposed. On a hydrophilic heated surface, at boiling incipience all the bubbles were lifted off the heated surface at atmospheric pressure and immediately collapsed in the subcooled liquid. On the contrary, when the surface was hydrophobic, bubbles mostly stuck on the nucleation sites at ONB condition. Furthermore, in this study, experiments were performed using rather hydrophilic and hydrophobic heated surface to propose the new mechanisms of NVG. An important result revealed in this work was that on a hydrophobic heated surface with high contact angle around 90°, bubble departure from all the nucleation sites which is a necessary condition to cause NVG, occurs in proximity to onset of significant void (OSV). The direct cause of OSV for the hydrophilic and hydrophobic surfaces was reattachment of lift-off bubble to heated surface, but bubble departure from nucleation sites was a good indication of OSV at hydrophobic surface.

One way to convey the occurrence of explosion under water and its effects on the structures is to use coning shock tube. By using a small explosive, this tube causes high pressure. In this essay, by suing LS-DYNA code, the explosion of a subsidiary amount of an explosive in the conning shock tube has been scrutinized. A numerical simulation has been done by using the MMALE (Multi Material Arbitrary Eulerain Lagrangian) solving method. To verify the validity of the selected method in software, first, the empirical tests performed by LeBlanc and Shukla is simulated. After assuring of the precision of the results, simulation of the desired problem is performed. In this research, first, the effect of the angel of the cone's head in the caused pressure inside the tube has been checked. Then, the operation of shock tubes with different lengths is checked. At the end, with the conversion of the weight of explosive, the study of the results and the reasons of the conversions in each parameter, a bond for the equivalent mass for all the shock tubes with different angels is represented and the bond for the present theory has been revised.

In this study, the mechanical properties of PP/Graphene and PP/Talc/Graphene hybrid nanocomposites compared. All samples were prepared by melt mixing technique using an internal mixer. The coupling agent between filler and matrix was Maleic anhydride grafted polypropylene. For PP/Graphene binary nanocomposites, by adding 0.75wt. % Graphene, elastic modulus, tensile and impact strength were increased. In higher amounts of Graphene, modulus was increased while tensile and impact strength were decreased. For PP/Talc/Graphene hybrid nanocomposites, adding 15% wt. Talc to PP caused an increase in elastic modulus and a decrease in tensile and impact strength. Furthermore, after adding Graphene to PP/Talc, elastic modulus, tensile and impact strength were increased. In higher amounts of Graphene (0.75 to 1.5% wt.), tensile and impact strength were decreased. The morphology of the samples was characterized by using Scanning Electron Microscopy (SEM) technique and crystallinity behavior was characterized by Differential Scanning Calorimetry (DSC). The SEM micrographs shows that there is no good adhesion between talc and PP matrix. Also from SEM, in higher amounts of Graphene, its plates stack together. The DSC result shows that talc has no considerable effect and Graphene has negative effect on the crystallinity. The results showed that hybrid nanocomposites containing 15% wt. Talc and 1.5% wt. Graphene had highest elastic modulus while PP/Graphene nanocomposites containing 0.75% wt. Graphene had highest tensile and impact strength.

In this paper, a hybrid model which consists of planar parallel robot with a 2-DOF arm is presented. This manipulator is based on cable parallel robots and a kinematic serial chain is utilized and added to a cable parallel chain. The hybrid manipulator can provide features of both serial and parallel mechanism. Understanding the orientability of the end-effector within this workspace gives a measure of the ability of the robot to perform manipulation tasks. Most underactuated parallel manipulators have a low rotational capability. To overcome it, this paper focuses its attention on a new family of serial-parallel manipulators. The initial goal of this design is to increase the orientability capability of a planar two-cable robot. The kinematic and dynamic analysis of this new type hybrid manipulator is presented. The dynamic modeling is performed by using a combination of Lagrange and Newton-Euler methods. This paper conducts the dynamic trajectory planning study to a novel design for the cable robots. In fact, time optimal trajectory planning is a strategy that is used to verify this new design. Two models are considered in the simulation part. A novel hybrid model is compared to a planar parallel cable robot. It is verified that the proposed design allows significantly reduced actuation energy and improved orientability capability compared to the parallel robot.

In this paper, Taguchi statistical method is implemented in the design of energy-absorbing composite shell structures with cylindrical geometry. Six energy-absorbing structure design parameters considered in this study are: geometric parameters including internal diameter, length and thickness; the other parameters are the stacking sequence of layers, fiber reinforcement type and manufacturing process. The first three parameters and the remaining ones have four and two levels respectively. So the orthogonal array L16 (4 ** 3 2 ** 3) was used for analysis of Taguchi. The purpose of design of experiment in this study was to maximize the amount of specific energy absorbed in the structure. The result shows that the stacking sequence of layers and geometry parameter include internal diameter and thickness had an effect on the opposite side, the other parameters had Minimal effect on specific energy absorbing. The first three parameters had most important role in design of energy absorbing structures. Another important result of this analysis was to determine the optimal characteristics of composite energy absorbing shells with stacking sequence of layers (90/0), internal diameter 63 mm, thickness 2 mm, vacuum bag molding process (VB), the fiber reinforcement type carbon and the length 160 mm.

The unsteady rotating force or dipole strength distribution, acting by the fan or propeller on the fluid, is predicted by inverse method. In this method, the far-field acoustic pressures are used in non-cavitating condition. In this paper, the far-field acoustic pressures are obtained from Ffowcs Williams and Hawkings (FW-H) equations using computational fluid dynamic (CFD) in specific hydrophone array and then the unsteady rotating force, acting by the propeller on the fluid, is obtained as the most important sound source in non-cavitating condition. The unsteady rotating forces are extracted using inverse method by analytical code in MATLAB. The correct solution is independence to the optimum select of regularization parameter from transfer function; the transfer function represents relationship between the force coefficients and the far-field acoustic pressure. Therefore, the appropriate range of regularization parameter should be choice in order to an ill-conditioned problem from transfer function is solve. The analytical code is solved for different regularization parameters and then the unsteady rotating forces are obtained for three sections on the blade surface. The inverse method could be used for dipole strength distribution calculation as the most important sound source in non-cavitating condition in order to design the noiseless of marine propeller.

Study on the mechanism of movement of animals has always been an important activity in academic researches. In recent decades the study of the motion of swimming and jumping in frogs and simulate the movements increased. on the work ahead by Modeled of how the study of the anatomy of frog leg muscle, a mechanism is designed to simulate the natural movement of the foot frog with just one operator. Provide a geometric approach to mechanism design, frog legs move according to the joint angles of the design. The major angular changes in the mechanism based on changing the angle of the frog's natural movement. Actuator has sinusoidal Circular motion and frog motion cycles can be created with different frequencies. Required frequency for Actuator obtains by given the speed linear motion of foot in a natural movement. Also by enter the drag force and the force generated by the foot frog swimming is simulated. Required torque for actuator is obtained by velocity and acceleration of motion. The results of simulations and compare them with results from the natural movement of the frog, represents the high resolution of mechanism to mimic the real motion of the leg and the frog swimming and matching the values of velocity and acceleration and the drag force and the axial force. According to the results, the designed mechanism can be used to simulation movement of different types of frogs in the swimming.

In order to assess the effect of turbulence models in prediction of flow structure with adverse pressure gradient, steady state Reynolds-averaged Navier-Stokes (RANS) equations in an annular axisymmetric diffuser are solved. After selection of the best turbulence model, an approach for the shape optimization of annular diffusers is presented. The goal in our optimization process is to maximize diffuser performance and, in this way, pressure recovery by optimizing the geometry. Our methodology is the optimization through wall contouring of a given two-dimensional diffuser length and area ratio. The developed algorithm uses the CFD software: Fluent for the hydrodynamic analysis and employs surrogate modeling and an expected improvement approach to optimization. The non-uniform rational basic splines (NURBS) are used to represent the shape of diffuser wall with two to ten design variables, respectively. In order to manage solution time, the Kriging surrogate model is employed to predict exact answers. The CFD software and the Kriging model have been combined for a fully automated operation using some special control commands on the Matlab platform. In order to seek a balance between local and global search, an adaptive sample criterion is employed. The optimal design exhibits a reasonable performance improvement compared with the reference design.

Nanotechnology has great potential applications in many fields such as chemistry, physics, material science, etc. In the recent years, due to the extraordinary properties of nanostructures, they are used in a wide range of nanodevices such as nanosensors, nanoactuators and nanocomposites. The effect of size on mechanical behavior of nanostructures whose size is comparable with molecule distances is important. Considering that classical continuum models are free scale and cannot capture the size effects, nonlocal continuum models are used for the analysis of mechanical properties of nanostructures. The nonlocal elasticity theory assumes that the stress at a reference point in the body depends not only on strain at that specific point, but also it depends on the strain at all other points. So, this theory contains long range interaction between atoms and internal scale length. This theory is capable to predict behavior of nanostructures without solving complicated equations. In the present work, the effect of considering small scale on the buckling of nanorings is studied. Governing equations are derived based on the nonlocal elasticity theory using the virtual displacement method and Hamilton's principle. Shear effect is achieved by Timoshenko beam theory. The governing equations are solved analytically. The effects of nonlocal parameter, radius, radius to thickness ratio and buckling mode number on the buckling loads of the nanorings are investigated.

A numerical and experimental study of the collapse and energy absorption behavior of thin-walled end capped conical shells under dynamic loading is presented in this paper. Among the structural components, the truncated conical shells whose energy absorption characteristics are better than others are used. In order to carry out the designed tests, a drop hammer machine has been used. Also in numerical part, Abaqus software capabilities have been applied. In this article, the effect of the velocity and mass of the hammer on the collapse behavior of these samples has been investigated. Moreover, by placing the cone reversely, the force effect on the collapse behavior evaluated and analyzed. Also, the multiple sets of cones as energy absorbing system are analyzed numerically. For the samples, mode of collapse of diamond with quadrilateral pattern was obtained and a very good agreement with experimental results was recorded. The results shows that the change of wall thickness has the most influence on the collapse behavior of these shells. So that with a 20% reduction of shell thickness, maximum force had 34.5% and the average force collapse 39.3% reduction.

In this paper, the algorithms for low frequency non-linear dynamic modeling and frequency model determining of LPRE is presented. Considerations that facilitate modeling and debugging processes is also investigated. Using of defined algorithms and also presented considerations is considered for a liquid propellant engine with oxidizer and fuel tanks. Describing equations of LPE is classified as many subsystems. Simulation is done in the SIMULINK environment of MATLAB software. Each simulated subsystem show one or more physical subsystem that their interaction is determined in LPE configuration modeling results demonstrate excellent dynamic behavior of LPE.Then SISO engine model in frequency domain is outcome based on resulted non-linear model of LPE using describing function. Frequency response code is developed for derivation of engine frequency model. Adequate frequency interval and input or excited signal amplitude are selected regarding LPE operating modes. In next step, frequency model is derived by stimulation of non-linear dynamic model with sinusoidal inputs includes considered amplitudes and frequencies. This subject is done by integration and engine output obtaining and Furrier integrals calculation at time that output get to steady state. Then system gains and phases calculation is done at the various amplitudes and frequencies for obtaining describing functions models. Frequency model evaluation characterized that can provide more efficient, simple and adequate conditions for analysis of LPE dynamics.

The viscous fingering instability of miscible non-Newtonian flow displacements in anisotropic porous media is studied. This instability was studied in a rectilinear Hele-Shaw cell and the shear-thinning character of the fluids has been modeled using the Carreau-Yasuda constitutive equation. In particular, the role of anisotropic properties of porous media including permeability and dispersion and also rheological parameters of non-Newtonian fluid is investigated through nonlinear simulation. In non-linear simulations, a spectral method based on the Hartley transforms are conducted and allowed to compare several non-linear finger interactions were observed in simulation. In this paper, three types of displacement are considered. In the first one, the displacing fluid and the displaced one are Newtonian and in the next two types of displacement, one of the displacing fluids or the displaced one is non-Newtonian. The evaluation of mixing length, sweep efficiency and transversely average concentration are examined for two different types of displacement where the displacing or the displaced phase were shear-thinning fluids and also for different anisotropic scenarios. The results indicate that in three types of displacement, the flow is more stable by increasing the anisotropic permeability ratio and also is more unstable by increasing the anisotropic dispersion ratio. Moreover, it’s concluded that in the case of the non-Newtonian fluid displaced the Newtonian fluid, by increasing the Deborah number and the power-law index, the more stable flow is obtained, while in the case of the Newtonian displaced the non-Newtonian one, the more unstable flow is obtained.

Residual stresses of those which remain in the material even after removing the entire external load. Residual stresses may be compressive or tensile depending on the type of the external loads. Compressive residual stresses improve the mechanical properties particularly the fatigue life of material. Compressive residual stresses can be induced by different techniques. Due to its easiness and low cost, deep rolling is one of the widely used techniques in industry to create compressive residual tresses. Deep rolling process is numerically simulated in this work and the effects of some important rolling parameters such as ball diameter, feed rate, penetration depth, and number of passes on the distribution of residual stress are investigated. Chaboche cyclic plasticity model is used in the simulations. The constants of the Chaboche model are calculated from the strain control cyclic tests on Al 7075. The results are validated using the experimental and numerical results reported in the literature. The results indicate that the depth and magnitude of the compressive residual stress increase with the ball diameter increase, depth of penetration and number of passes. Also, the value of residual stress and its uniformity decrease by increasing the feed rate. In addition, Chaboche cyclic plasticity model can simulate material behavior in a low cycle loading such as deep rolling and using finite element method instead of experimental methods for measuring residual stresses reduces cost and time of solution and reveals more depth of residual stress distribution.